Whole House Fan Shutter - 30" - Interior Use - White Injection molded frame with smooth white aluminum louvers - Interior Use Automatically open when fan turns on, & close when fan turns off to keep out air, insects, and dust. Balanced shutters for complete automatic service-free operation. Installs centered on joist w/18" or 24" centers w/2 cut openings, 1 each side of center joist Opening Size Required: 33.75" x 28.25"